Gas chromatography is essentially a method of separation of mixtures of substances into their individual components. In a typical analysis of a sample by a gas chromatograph, the sample is introduced into a chromatographic column together with a carrier gas. At the end of the column the individual components are more or less separated in time. Generally, detection of the gas provides a time-scaled pattern which, by calibration or comparison with known samples, indicates the constituents of the test sample.
Separation of the sample usually occurs within the column upon interaction with the stationary phase, whereby the distribution coefficients of the elements may cause the separation. Typically, the constituents of a test sample in a carrier gas are adsorbed and desorbed by a stationary phase material in a column. Polarity may also play a role in separating the components from one another over time. Differences in polarites may cause the components to attach to the stationary phase at different intervals. Elements exiting the column are typically detected by a detector and the results are usually charted, often resulting in a chromatogram.
For gases that may have difficulty being detected by the detector, the system for the gas chromatography may optionally include a reactor, which generally heats the desired gas with a reactant to form a detectable compound. The reactant may be a gas, liquid, or solid and varies according to the desired gas to be detected. Typical reactants include air, hydrogen, and oxygen. A detectable compound is one that generally provides an electrical signal detectable by the detector.
Typical detectors for measuring the gases exiting the column include mass spectrometers and electrolytic conductivity detectors. Other detection systems include thermal conductivity, flame ionization and argon detectors. Electrolytic conductivity detectors usually provide an electrical signal that is functionally related to the presence of a selected element.
Electrolytic conductivity detectors are known for investigating the properties of electrolytes in solutions. Such devices typically include electrode surfaces with a continuous phase liquid electrolyte therebetween. These detectors may entail measuring a difference in resistance in the electrolytic material before and after the gas exiting the column enters the detector and is absorbed by the electrolytic material. If the gas was mixed with a reactant in the reactor, the reactant may also be absorbed in the electrolytic material before providing a detectable electrical signal. A possible disadvantage of the conductivity detector is that absorption by the electrolytic material takes time, which lengthens the detector's response time. The disadvantage may be exacerbated if both the gas and reactant need to be absorbed. Another possible disadvantage is the limited accuracy of the detector. Because the gas is detected indirectly, where the difference in resistance of the electrolytic material may indicate the type and/or concentration of the gas, a standard of deviation in the measurement error between the electrolytic material measurement and correlation from this measurement to the gas may negatively affect accuracy.
A typical conductivity detector is described in U.S. Pat. No. 4,440,726 to Coulson and shown in FIG. 1. As shown, an electrolyte, reactant gas, and gas exiting from the column enter the capillary. Electrodes 24 and 28 are placed in the electrolyte solution which may measure the difference in resistance.
Similar to the conductivity detector, the mass spectrometer and other detection systems of gas chromatography have potentially limiting abilities to detect gas with a high degree of sensitivity. As mentioned in U.S. Pat. No. 6,165,251 to Lemieux et al., gas chromatography systems in general have insufficient sensitivity to measure amounts of volatiles in the parts per billion concentration range.
What is desired, therefore, is a gas chromatography system having a detector with improved sensitivity. Another desire is provide a gas chromatography system that detects gas in the parts per billion concentration range. What is also desired is a gas chromatography system having an improved response time.